Inhibition by cromakalim, pinacidil, terbutaline, theophylline and verapamil of non-cholinergic nerve-mediated contractions of guinea-pig isolated bronchi

Modulation of sensory nerve function in the airways

Several clinical studies document a greater discrimination between asthmatic and healthy subjects in bronchial responsiveness to a range of stimuli such as cold air, distilled water and sodium metabisulphite, than to conventional bronchoconstrictor agonists including histamine and methacholine. One of the mechanisms thought to account for the bronchoconstriction induced by these agents is via reflex activation of the cholinergic pathway. An increase in sensory nerve (afferent) activity in asthma might account for the increased responsiveness to these agents. If so, a number of strategies are available to inhibit the function of afferent nerves which could lead to a suppression of bronchial hyperresponsiveness, including (1) inhibition of afferent activity, (2) inhibition of neuropeptide release and (3) antagonism of tachykinin receptors. As there are numerous reviews dealing with the latter, in this review Domenico Spina, Saloni Shah and Selena Harrison focus on the first two strategies.

Effects of five different airway smooth muscle relaxants on inhibitory neurotransmission in isolated guinea-pig trachea in vitro

Pharmacodynamic effects produced by terbutaline (10 nM), theophylline (10 microM), sodium nitroprusside (30 nM), levcromakalim (0.3 microM) or isradipine (1 nM) on frequency-dependent relaxations induced by electric field stimulation of either proximal or distal parts of isolated guinea-pig trachea were studied in vitro. Preparations were depleted for tachykinins by capsaicin, pretreated with atropine (0.1 microM) and contracted by histamine (2 microM). Drug effects were studied in preparations with combined adrenergic and inhibitory non-adrenergic non-cholinergic (NANC) innervation and in preparations with inhibitory NANC innervation either with or without additional treatment with N(G)-nitro-L-arginine methyl ester (L-NAME) (100 microM). In preparations with combined adrenergic and inhibitory NANC innervation terbutaline, sodium nitroprusside, levcromakalim and isradipine significantly reduced relaxant responses to electric field stimulation in proximal preparations, whereas distal preparations were only affected by terbutaline. In preparations with inhibitory NANC innervation without L-NAME pretreatment, terbutaline significantly enhanced relaxant responses to electric field stimulation only in distal preparations, whereas theophylline, sodium nitroprusside and levcromakalim significantly augmented responses to electric field stimulation in both proximal and distal preparations. In preparations with inhibitory NANC innervation pretreated with L-NAME, theophylline significantly inhibited relaxant responses in distal preparations, whereas sodium nitroprusside, levcromakalim and isradipine significantly augmented relaxant responses to electric field stimulation in proximal preparations. It was concluded that drugs used in the present study can modulate the effects of inhibitory autonomic and NANC neurotransmission in isolated guinea-pig trachea. Furthermore, it was shown that some variation in drug effects exists in relation to proximal and distal parts of guinea-pig trachea.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.